It is known that pavers are used to lay courses, for example in road construction, using what are known as floating screeds. Here, the screed is pivotably mounted centrally on the paver by means of two tow arms located outside the paver frame or chassis, i.e., it is towed and adjusted in terms of height. The screed itself floats on the course to be laid at a positive angle of attack, i.e., the front edge of the screed in the direction of travel is situated at a higher position than the rear edge. The positive angle of attack results from parameters such as load-bearing capacity of the mix, tamper and vibration compaction, weight of the screed, paving speed, etc.
This positive angle of attack, and in particular also the metering slope of the tamper, which tamper is situated in the front region of the screed, form “ramps” when seen in the direction of travel. However, given sufficiently high mix temperatures, these “ramps” are compressed by the compaction elements, i.e., at least one tamper and a vibration assembly, and by the weight of the screed down to the height predetermined by the rear edge of the screed, which operation is also termed compaction. If then, as a result of pauses during paving, for example, the mix lying under the screed and in front of the tamper becomes cooler, this means that the mix can be compacted significantly less well.
The consequence of this is that, when the paver is started up again for the purpose of continuing the paving operation, the screed will deflect upward on the ramps and will only reassume its intended height, which lies at a lower height level than said ramps, once it has reached mix of normal temperature. The start-up humps become higher the longer a pause for charging, and thus the effect of cooling, last.
In addition, such start-up humps are further promoted, for example, in connection with high-compaction screeds and/or the use of stiff bitumen, which today is a feature of conventional paving practice.
These start-up humps constitute raised uneven areas which in some cases considerably exceed the permissible unevenness. It is therefore attempted to eliminate the start-up humps through manual activity by means of rakes, etc. Apart from the increased costs, the planarity achieved through manual activity is inferior to that which can be achieved with a satisfactorily operating paver.
During the paving operation, the screed, which is articulated centrally on the paver via tow arms and tow points, is towed by said paver and altered in its vertical position. Screed transport cylinders, which are situated in the rear region of the paver and which raise the screed for the purpose of transportation, are in a pressureless state, i.e., one which does not influence the vertical position of the screed, during the paving operation. These screed transport cylinders are fastened by their piston side to the upper rear frame of the paver and by their piston rod side to the tow arms connected to the screed.
In order to counteract start-up humps, the screed transport cylinder is blocked on the piston side for a few seconds at the moment of restarting, with the result that the screed cannot deflect upward, since now the paver stops with its weight against said screed. The duration of blocking is determined such that it is ensured that the paver has overcome the region of the cold mix lying under the screed and in front of the tamper.
However, since the screed transport cylinders are situated within 2.5 or 3.0 m, depending on the basic width of the paver, the action of what is referred to as the screed elevation locking is satisfactory in the central region of a screed but, because of the elasticity of the screeds, not in the outer region. It should be noted in this connection that extendable screeds have working widths of up to 9.0 m and screeds which can be built on manually have working widths of, in some cases, up to more than 13 m.
In the case of screeds which can be built on manually, it is attempted to increase the vertical rigidity of the screed in itself, for example by means of supports situated above the screed. However, this is only partly successful since, owing to the large screed width, the supporting forces are not sufficiently large to ensure the action of the screed elevation locking in the outer region of the screed as well.
However, the situation is particularly critical in connection with extendable screeds. In this case, as is known, extendable screed-widening parts (also called extendable screeds) situated behind the basic screed are extended hydraulically and, according to the particular requirement, widened out up to 9.0 m by means of prolongations which can be built on manually. The mode of action of the screed elevation locking with respect to the basic screed is satisfactory in this case too. However, it is already significantly minimized as a result of play and elasticity in the guide mechanism of the extendable screeds, especially as, in this region, no supporting means can be used as in the case for the screeds which can be built on manually. Even if that occurred, as already mentioned in the case of the screeds which can be built on manually, it would not be sufficient.